When do I need to use a JavaScript framework such as backbone.js/AngularJS? And why?

Angular is much more then the backbone and so the learning curve, if you looking for SPA (Single Page Application) with some interactions similar to linked mobile (they have been using backbone.js too) then go for it. Angular has a high wow factor at the beginning. It can do some amazing things - like two-way bindings - without having to learn much. And it looks quite easy at first sight. But after you have learnt the very basics it is quite a steep learning curve from there. It's a complex framework with lots of peculiarities. Reading the documentation is not easy as there is a lot of Angular specific jargon and a serious lack of examples. The basic of Backbone are quite easy to learn. But soon you find that there are not enough opinions there to know how to best structure your code. You will need to watch or read a few tutorials to learn some best Backbone practices. You can find few comparisons below in terms of features. Data-binding The most prominent feature that separates the two libraries is in the way models and views are synchronized. Whereas AngularJS supports two way data-binding, Backbone.js relies heavily on boilerplate code to harmonize its models and views. REST Backbone.js communicates well with RESTful backends. A very simple use of REST APIs is also available with AngularJS using the $resource service. AngularJS also provide a $http service which is more flexible, connecting to remote servers either through a browser's XMLHttpRequest object or via JSONP. Templating AngularJS templating uses a combination of customizable HTML tags and expressions. Backbone.js uses different templating engines such as Underscore.js. It's again the choice which one needs to make and depending upon the comfort level to choose the one of them.

Good for different feature and provide good and smooth GUI without any dependencies

javaScript improve pages from validation, styling, finally it's for page actions can't be used for databases ..

For Large Scale Web Application and sometimes most of these frameworks already have business support for most of your task, i use the Dojotoolkit for most of my work and the last thing i want to be bothered about is writing validation from scratch or even tree map or anything i can easily Dojo, and now because Web application tend to be complex you need a framework that ensures modularity, extensibility and also Decoupling. its really vital